Cryogenic flat-panel gas-gap heat switch

doi:10.1016/j.cryogenics.2016.07.006

Cryogenics

Volume 78, September 2016, Pages 83-88

https://doi.org/10.1016/j.cryogenics.2016.07.006 Get rights and content

Highlights

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A novel compact flat-panel gas-gap heat switch operating at 100–120 K is realized.
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Selective laser melting additive manufacturing process is used.
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A single-side guarded hot plate apparatus is used to characterise the heat switch.

Abstract

A compact additive manufactured flat-panel gas-gap heat switch operating at cryogenic temperature is reported in this paper. A guarded-hot-plate apparatus has been developed to measure the thermal conductance of the heat switch with the heat sink temperature in the range of 100–180 K. The apparatus is cooled by a two-stage GM cooler and the temperature is controlled with a heater and a braided copper wire connection. A thermal guard is mounted on the hot side of the device to confine the heat flow axially through the sample. A gas handling system allows testing the device with different gas pressures in the heat switch. Experiments are performed at various heat sink temperatures, by varying gas pressure in the gas-gap and with helium, hydrogen and nitrogen gas. The measured off-conductance with a heat sink temperature of 115 K and the hot plate at 120 K is 0.134 W/K, the on-conductance with helium and hydrogen gases at the same temperatures is 4.80 W/K and 4.71 W/K, respectively. This results in an on/off conductance ratio of 37 ± 7 and 35 ± 6 for helium and hydrogen respectively. The experimental results matches fairly well with the predicted heat conductance at cryogenic temperatures.

Introduction

We previously reported the development of a compact flat-panel heat switch produced with selective laser melting additive manufacturing technology and operating at 295 K [1]. The heat switch is symmetric along the flat faces i.e. the device plates as shown in Fig. 1. The outer dimensions of the heat switch are 11 cm × 11 cm and the thickness is 3.2 mm. Between the device plates, the extended surfaces are structured to increase the heat transfer surface of the gas. The gap between the two surfaces is about 200 μm. The internal surface area of the fin structure is approximately four times that of the functional area of the device plate (10 cm × 10 cm). The length and width of the fins is 1600 μm and 250 μm respectively and they run along the entire length of the device plate. The device plates serve as the contact areas with a heat source or sink. The two device plates are held together by four side covers and 49 evenly spaced support pillars. The diameter of each support pillar is 250 μm and the height is equal to the gas-gap spacing equal to 200 μm. Three of the four side covers have a serpentine shape (edges A, B and C, in Fig. 1). This shape was chosen to maximize the length of heat transfer path from one device plate to the other. This serpentine shape was not possible for the fourth side cover due to the overhanging geometry design constraint of additive manufacturing technique. Instead an elliptical side cover is used. This side cover houses the two feed-throughs, to let gas in and out of the gas gap. Support pillars are connected from the top of fin to the opposing device plate to increase the structural integrity of the device. The heat switch is produced from Ti-6Al-4V powder. This device is characterized at room temperature (295 K) and the measured off-conductance is 0.2 W/K and the on-conductance with hydrogen as working gas is about 7.6 W/K [1].

The main objective of this paper, is to explore the flat-panel gas-gap heat switch at cryogenic temperature conditions. The unique characteristics of this heat switch type is the compact shape (flat-panel) and a relatively high on-conductance at cryogenic temperature. Another key advantage of the additive manufacturing technology is the three dimensional flexibility in the design of outer structure so as to tailor the heat switch to other geometries, namely flat, cylindrical or other shape. Several novel thermal control strategies at cryogenic temperature are possible with this type of heat switch, to name a few; in separating the cooling stages in a cooler chain, in controlling the heat flow to a sorption compressor [2] and in heat-capacity measurements of materials [3].

In this paper, we will briefly describe a simplified thermal model of the gas-gap heat switch. The influence of the temperature variation of the thermal conductivity of the gas and the materials is discussed, followed with the estimation of heat conductance in the off- and the on-states. An apparatus for measuring the heat conductance of the heat switch at cryogenic temperature is described, followed by the measurement results. The heat switch is characterized with helium, hydrogen and nitrogen as working gas.

Section snippets

Design

The heat conductance of the gas-gap heat switch varies with the gas pressure in the gas gap. The heat switch can be classified in the following three operating regimes.

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Off-state
In the off-state, the heat conductance is mainly due to the structural parts that link the two device plates and the radiation between the surfaces. A first order thermal model representing various resistances of the heat switch in the off-state is shown in Fig. 1 (v).
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Active regime
In this regime, the heat transfer in the

Set-up

The apparatus to characterize the heat switch at 295 K is described in our earlier work [1]. The same apparatus cannot be used at cryogenic temperatures because the parasitic heat leak to the hot plate will be significant. Fig. 4 shows the schematic of test rig with a single-sided guarded hot-plate, one specimen and one cold plate.

The cold plate consists of a 100 mm × 100 mm × 10 mm copper plate. The cryocooler is attached to the bottom of the copper plate with a braided copper thermal link. A heater

Results

In the measurements at 295 K, a commercial heat sink compound is smeared on both contact sides of the heat switch to make a good thermal contact with the hot and cold plates of the test rig. We observed that the same material is not suitable at cryogenic temperature range. The measured on-conductance (see Table 2) at a cold plate temperature of 110 K is 0.546 W/K, which is an order of magnitude lower than the expected value (see Fig. 3). The inspection of the heat-switch surface after the

Conclusions

We have demonstrated a compact flat-panel gas-gap heat switch operating at cryogenic temperature. The tests indicate that an on/off ratio of 35 can be realized with hydrogen as the working gas, at a cold and hot plate temperature of 115 and 120 K respectively. Up-scaling the frontal dimensions of the heat switch will increase the on/off ratio because the parasitic losses due to the side covers scale with perimeter. ZrNi is a suitable sorber material with hydrogen as the working gas for the

Acknowledgement

The authors acknowledge lab assistance of Cris Vermeer and Harry Holland.

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Research paperCryogenic flat-panel gas-gap heat switch

Highlights

Abstract

Introduction

Section snippets

Design

Set-up

Results

Conclusions

Acknowledgement

Phys. Procedia

Vacuum

J Alloys Compd

Research paper
Cryogenic flat-panel gas-gap heat switch